1. Field of the Invention
The present invention relates to a permanent magnet type printing head for an impact dot printer.
2. Description of the Prior Art
In prior art systems, a printing head as shown in FIG. 1 is generally used, wherein yokes 2 and 3 are respectively mounted on and beneath a permanent magnet 1, one yoke 2 being provided with a plate spring hammer 5 having a hammer tip 4 disposed on the upper surface of the extreme end thereof, the other yoke 3 being provided with a magnetic pole 7 having a releasing electromagnetic coil 6 wound thereon, whereby during waiting, the hammer 5 is attracted by the permanent magnet as shown and during printing, the electromagnetic coil 6 is energized to release the hammer 5.
In the aforementioned prior art printing head, however, since the plate spring constituting the hammer 5 comprises a magnetic path, the plate spring cannot be greatly reduced in thickness, and thus, the inertia of the hammer 5 is inevitably high, thereby failing to provide a high speed operation.
An improved printing head as shown in FIG. 2 has been proposed in which, an arm 2a is provided with a through hole 8 between the hammer 5 and the magnetic pole 7 and is extended from the upper yoke 2, and the hammer 5 has an armature 9 which is movable within the through hole 8 through a small clearance a.
In the aforementioned improved printing head, a main magnetic flux of the permanent magnet 1 flows from the yoke 2 into the magnetic pole 7 through the small clearance a and armature 9, and the plate spring hammer 5 does not comprise the main magnetic path. Thus, the plate spring hammer 5 may be reduced in thickness and therefore, the inertia of the hammer 5 can be decreased to thereby render the high speed operation possible.
However, the printing head of this type has the following disadvantages:
(1) Large amount of wear in the armature 9 and in the head of the magnetic pole
In the printing head of this type, the armature 9 hits the magnetic pole 7 as a result of the primary vibration (vertical vibration) of the plate spring hammer 5 during the printing operation, and in addition, a lateral oscillation movement of the bottom surface of the armature 9 occurs as a result of the secondary vibration (vertical vibration of the hammer tip portion) due to the mass of the hammer tip portion after the attraction of the armature to the pole; the secondary vibration causes friction of the upper surface of the magnetic pole, the amount of wear of which is not regligible.
(2) Dimensional precision of the armature 9 is hard to obtain, resulting in a higher cost.
For printing heads having a number of aligned hammers, such as printers for Chinese characters, it is necessary to make constant the flexture amount of the plate spring (hammer 5) during waiting and the stroke of the hammer during printing, in order to provide for printing at high speeds and in a stabilized fashion. That is, the hammer used must be constant in height from the fixed surface of the hammer to the bottom surface of the armature and in height from the bottom surface of the armature to the extreme end of the hammer tip, and thus considerable care must be taken to the management of the dimensions in terms of the manufacture of the printing head, resulting in a higher cost.